Dental implants are inserted into the base of the jaw and have a portion protruding through the mucous gum tissue for providing attachment anchorage for artificial teeth. An artificial tooth may be attached directly to the implant, or to an abutment connected to the implant. Dental implants are becoming increasingly popular alternatives to dentures.
Dental implants have been used in the hard bone of the lower jaw with great success. The upper jaw bone, however, is very soft and it is common for dental implants in the upper jaw to lack primary stability.
There are three major types of dental implants: 1) press-fit; 2) self-tapping; and 3) pre-tapping. The press-fit type implants are inserted into holes drilled into the bone without rotation. Press-fit implants do not couple tightly to the soft bone of the upper jaw.
Self-tapping and pre-tapping type implants have horizontal threads for being screwed into holes drilled into the jaw-bone, for better mechanical coupling to the jaw bone. Self-tapping implants create grooves in the hole as the implant is screwed in. Prior to the insertion of pre-tapped implants, a tool is used to form threads in the hole. While screwing self-tapping and pretapping implants into the bone generally improves the mechanical coupling between the implant and bone, it has been found that in the soft bone of the upper jaw, the rotation of both the self-tapping and pre-tapping implants may destroy the grooves, preventing tight coupling between the implant and the bone.
After insertion of the implant, a sufficient period of time must be given for the tissue to heal and for the bone to grow sufficiently around (and sometimes through) the implant for the dental implant to become securely engaged in the jaw. This typically requires about three months. An artificial tooth is then attached to the implant, directly or by attachment to an abutment attached to the implant.
Primary stability, osteointegration and the aesthetic appearance of the implant and prosthetic tooth in the mouth are important considerations in the design of a dental implant. Mechanical solutions to achieving primary stability and osteointegration have generally ignored the natural shapes of the root and crown of the natural tooth being replaced and have not conformed to the natural relationship between the tooth and the jaw bone, resulting in adverse immunological responses by the jaw bone which both weakens primary stability and osteointegration, and sacrifices aesthetics.
FIG. 1 shows two adjacent natural adult teeth 100, in the upper jaw. A tooth 100 has a crown and a root. The crown may be divided into a cervical ⅓, a middle ⅓ and an incisal ⅓. The root may be divided into a cervical ⅓, a middle ⅓ and an apical ⅓. The interface between the cervical ⅓ of the tooth and the cervical ⅓ of the root is referred to as the cemento-enamel junction. It has been observed that in a natural tooth, the distance d3 between the cemento-enamel junction 104 and the crest 102 of the jaw bone is typically about 1.8 mm and the distance d4 between the crest 102 of the jaw bone and the gingival margin 108 is typically about 3 mm.
Adverse immunological responses may be caused by a variety of stimuli. For example, it has been found that the failure of interface between the dental implant and the prosthetic tooth to conform to the shape and location of the cemento-enamel junction between the crown and root of the tooth being replaced causes an adverse immunological response by the bone to the dental implant, resulting in bone resorbtion. This can weaken the bond between the implant and the bone and leave pockets which can collect plaque. In addition, since a constant distance is naturally maintained between the bone crest and the gingival margin, as the bone resorbs, the gingival margin recesses, presenting an unpleasant aesthetic appearance.
Another cause of adverse immunological responses in the jawbone are microgaps between the dental implant, whose top surface is typically positioned at or below the bone crest, and the abutment attached to the implant. Once again, bone resorbtion and gingival margin recession may result.
Some dental implants, such as the ITI(R) implants from the Straumann Company, Waltham, Mass., position the top surface of the implant above the bone crest. The jaw bone is not, therefore, exposed to microgaps between the implant and an abutment, decreasing adverse immunological responses. However, the flat tops of these implants do not match the shape of the cemento-enamel junction of the natural tooth being replaced. Bone resorbtion and the resulting recession of the gingival margin still occur, particularly interproximally.
Dental implants are also typically cylindrical. However, teeth are not so regularly shaped. While attempts have been made to conform the abutment and the prosthetic tooth to the natural shape of the cervical ⅓ of the root and the cervical ⅓ of the crown of the tooth being replaced, the unnatural shape of the dental implant limits how closely the natural shape of the root and crown can be recreated. Voids are therefore present between the dental implant and the bone of the jaw, and between the prosthetic tooth and the gingivus, which can allow for the growth of soft tissue and the collection of plaque. Such soft tissue may interfere with the osteointegration of the implant, resulting in implant failure. Attempts have been made to prevent soft tissue growth by filling the gaps between the implant and the jaw bone by artificial bone or by covering the interface between the gums and the implant with a membrane. Such designs have an increased risk of infection and lack osteointegration. Rotation of the implant, either during rotation of the implant to insert the implant into the jaw bone or after final positioning of the implant, also interferes with attempts to close such gaps.